Various transmission formats have been explored in order to increase the capacity of fiber optical communication systems. One such format is Differential Quadrature Phase Shift Keying (DQPSK), in which data is encoded as a difference in phase of a transmitted optical signal.
FIG. 1 illustrates a conventional DQPSK modulator. Modulator 100 includes a laser 110, which may be a distributed feedback (DFB) semiconductor laser operating in a continuous wave (CW) mode and at a particular wavelength. Light output from laser 110 is supplied to a splitter 112, which provides substantially half the received light to inner Mach-Zehnder (MZ) modulator 114 and half the light to inner MZ modulator 116. Inner MZ modulator 114 includes first and second arms 112-a and 112-b, each of which being coupled to corresponding electrodes 112-c and 112-d. Inner MZ modulator 116 includes first and second arms 112-e and 112-f coupled to corresponding electrodes 112-g and 112-h. 
As further shown in FIG. 1, precoder circuits 116 and 118 are provided which receive data streams DATA1 and DATA2, respectively. Precoder circuit 116 outputs in-phase signal I and its complement I(bar) and precoder circuit 118 outputs quadrature signal Q and its complement Q(bar). I, I(bar), Q, and Q(bar) are supplied to electrodes 112-c, 112-d, 112-g, and 112-h, respectively. Light passing through arms 112-g and 112-h is combined and the polarization of the combined light is rotated by π/2 (see rotator 118). The rotated light is then combined with light output from arms 112-a and 112-b. 
Signals I and I(bar) and Q and Q(bar) serve to drive inner MZ modulators 114 and 116 in a push-pull fashion, respectively. As such, electrodes 112-c and 112-d are driven π out of phase with respect to one another. Likewise, electrodes 112-g and 112-h are also driven π out of phase with respect to one another. Accordingly, relatively high voltages must be applied to maintain such a phase difference between the signals applied to the electrodes of the inner MZ modulators. These high voltages require higher power and render the conventional DQPSK modulator incompatible with lower power technologies, such as CMOS.
Moreover, although an excess voltage is applied in order to achieve the π-phase shift imposed by electrodes 112-g and 112-h, a portion of this phase shift is retracted by π/2 rotator 118. Accordingly, an additional voltage is applied through rotator 118 to remove part of the π-phase shift. Modulator 100 is thus inefficient in this respect.
It is accordingly a primary object of the invention to provide a modulator that has increased efficiency.